Satellite communication systems have customarily employed multi-reflector antenna architectures, often of center-fed Cassegrain configuration, in order to optimize the collection of electromagnetic energy within a prescribed frequency band transmitted over relatively long distances (e.g., earth station-satellite-earth station). Where the number and size of antenna components is not necessarily a major concern, such as a fixed, land-based facility that has ample room for the placement of one or more relatively large structures, it is common practice to employ a relatively large main reflector, and an associated subreflector that is on the order of several tens of wavelengths in diameter. Because of the substantial blockage associated with such a subreflector, the diameter of the main reflector may be in excess of five meters in diameter at C and/or X band. While such a large dimensioned subreflector--main reflector structure is capable of successfully performing its intended functionality for a given operational frequency band, if the earth station is to provide communication capability at separate bands, additional subreflector--main reflector pairs configured for operation at those bands must be installed.
In contrast, many communication systems, such as shipboard-mounted facilities, have only a limited amount of space for the installation of antenna components. In such spatially constrained environments, where antenna size is limited and its directivity pattern must typically comply with a very strict specification, it is not practical to install even one, much less multiple spatially large reflector structures. One proposal to deal with this space constraint problem is to employ a ring focus antenna, having a parabolic main reflector and a `shaped` (i.e., ellipsoid) subreflector.
Advantageously, the conical properties of the ellipsoid-shaped subreflector provide a dual focus characteristic, with one of its foci displaced toward the vicinity of the aperture of the main reflector where a feed horn is installed. The other focus is symmetric about the antenna axis in the form of a ring, which enables the antenna to obtain a substantially uniform amplitude distribution in the aperture plane. As a consequence of this geometry characteristic, the antenna can is more compact than a conventional center-fed structure.
For non-limiting examples of documentation detailing the configuration and operation of a conventional ring focus antenna, attention may be directed to the following publications: "Amplitude Aperture-Distribution Control in Displaced-Axis Two-Reflector Antennas," by A. Popov et al, Antenna Designer's Notebook, IEEE Antennas and Propagation Magazine, Vol. 39, No. 6, Dec. 1997, pp. 58-63; "The Theoretical Analysis of Shaped Dual-Reflector Antenna with Ring Focus," by T. Wang et al, Conference Proceedings, 20th European Microwave Conference 90, pp 1553-1558; "Shaped Dual-Reflector Antenna with Ring Focus," by R. Zhang et al, Science in China (Series A) Vol. 34, No. 10, Oct. 1991, pp 1243-1255; "Two-Reflector Antenna," by Y. Erukhimovich et al, Radio Research Institute, Ministry of Posts and Telecommunications, USSR, pp. 205-207; and the Canadian Patent to Schwarz, No. 1,191,944, entitled "Improved Shifted Focus Cassegrain Antenna With Low Gain Feed," and assigned to the assignee of the present application.
Now although a ring focus antenna, such as those described in the above literature, is intended to provide reduced subreflector blockage and thereby the overall size of the antenna structure to be smaller than a conventional Cassegrain architecture, its ellipsoid-shaped subreflector is still on the order of several tens of wavelengths in diameter, and is spaced apart from the antenna feed (horn) by similar electrical distance.
To minimize subreflector blockage, the size of the main reflector is still substantial; at C or X band, the main reflector may have a diameter on the order of three meters, depending upon gain and sidelobe requirements. This means that in order to provide communication capability at multiple spectrally separated bands, such as at each of C band and X band, the overall size of two ring focus antenna structures may extend to a diameter on the order of 16-20 feet. This not only places a strain on the space limitations of a facility such as a shipboard-mounted satellite communication system, but does not solve the hardware complexity and cost problems of having to install a separate ring focus pair for each operational band.